Advances in technology have resulted in smaller and more powerful computing devices. For example, there currently exist a variety of portable personal computing devices, including wireless computing devices, such as portable wireless telephones, personal digital assistants (PDAs), and paging devices that are small, lightweight, and easily carried by users. More specifically, portable wireless telephones, such as cellular telephones and Internet protocol (IP) telephones, can communicate voice and data packets over wireless networks. Further, many such wireless telephones include other types of devices that are incorporated therein. For example, a wireless telephone can also include a digital still camera, a digital video camera, a digital recorder, and an audio file player. Also, such wireless telephones can process executable instructions, including software applications, such as a web browser application, that can be used to access the Internet. As such, these wireless telephones can include significant computing capabilities and may support increasing wireless communication capability, particularly in downlink communications that provide information to the wireless telephones.
Such computing devices typically include circuitry that may exhibit non-ideal characteristics. For example, resistors, such as polysilicon resistors (“poly-resistors”), exhibit changes in conductivity (“conductivity modulation”) based on an amount of applied voltage. A poly-resistor may be formed of a resistive strip over a semiconductor substrate, such as over a doped region or “well” in the substrate. The poly-resistor may be formed on a dielectric layer that electrically isolates the resistive strip from the substrate. Applying a voltage across the poly-resistor (e.g., by applying different voltages to a first terminal coupled to a first end of the resistive strip and a second terminal coupled to a second end of the resistive strip) induces current through the poly-resistor. The induced current may be approximated as V=IR according to Ohm's Law, where V is the voltage across the poly-resistor, I is the current through the poly-resistor, and R is the resistance of the poly-resistor. Although Ohm's Law may provide a satisfactory approximation of the induced current under conditions in which the resistance of the resistor is approximately constant, under other conditions the approximation of V=IR may be unsatisfactory, such as for a resistor having a resistance that varies based on the voltage applied across the resistor.
Applying the voltage V across the poly-resistor also causes charge accumulation in the poly-resistor and charge depletion in the substrate (or charge accumulation in the substrate and charge depletion in the poly-resistor, depending on the voltage). The charge accumulation on one side of the dielectric layer and charge depletion on the other side of the dielectric layer results in a parasitic capacitance between the resistive strip and the substrate. This parasitic capacitance affects the conductivity, and therefore the resistance, of the resistive strip. As another example, when a conductive element, referred to as a “shield,” is positioned above a top surface of the resistor, applying a voltage across the terminals of the resistor may also result in a parasitic capacitance between the shield and the resistor.
Changes in the conductivity of a resistor cause the resistor's current-voltage characteristic to be non-linear (e.g., the resistor has a resistance that changes as a function of voltage applied across the resistor). When such resistors are used as feedback resistors of an amplifier, the conductivity modulation introduces distortion at the amplifier's output. For example, conductivity modulation at a resistor coupled to a headphone power amplifier degrades the quality of an audio signal output of the headphone power amplifier.